Certain photolithography parameters, such as, the variation of critical dimension (“CD”) printed with pitch, otherwise sometimes referred to as Optical Proximity Effect (OPE), such as, in a scanner imaging system, shows a behavior that is characteristic of the imaging and process conditions and is sensitive to variations in those conditions. Maintaining stable process conditions can improve Optical Proximity Correction (OPC) used to offset the effects of OPE. One of the factors which affects the OPE is spectral bandwidth. Passive bandwidth stabilization techniques have been effective in meeting OPE control requirements. However, future tighter OPE specifications, among other things, will require advanced active bandwidth control techniques. Such active control techniques not only improve the stability of E95 bandwidth, but also regulate E95 bandwidth to a desired setpoint (i.e., within a selected very narrow range).
A variable magnification line-narrowing module is described in U.S. Pat. No. 6,393,037 which issued to Basting et al. on May 21, 2002 (“Basting”), the contents of which are hereby incorporated by reference herein. The abstract of Basting describes a tunable laser including an angular dispersive element and a beam expander including one or two rotatable prisms along with a grating in a line narrowing module to adjust the bandwidth resulting from adjusting the magnification of the beam incident on the dispersive element. The prism beam expanders, when two are used, are disclosed to be mechanically linked to so that the angle of incidence of the beam on the dispersive element is not changed when the magnification changes. This arrangement makes it very difficult, if not impossible, to control center wavelength as well as bandwidth utilizing the rotatable prisms.
Rafac, “Overcoming limitations of etalon spectrometers used for spectral metrology of DUV excimer light sources”, Optical Microlithography XVII, Bruce W. Smith, Editor, Proceedings of SPIE, Volume 5377 (2004) pp. 846-858, the disclosure of which is hereby incorporated by reference, discusses methods and apparatus for calculating bandwidth, such as E95 bandwidth.
GigaPhoton is believed to advertise a product that performs E95 control using some sort of optical actuation. Japanese Published Patent Application 2006024855, published on Jul. 9, 2004, discloses a variable magnification LNM, also with two rotatable prisms and the use of a differential discharge timing for bandwidth control. Such an arrangement makes it difficult, if not impossible, to control center wavelength and bandwidth with the prisms. Japanese Published Patent Application 2004311766, TWO-STAGE LASER DEVICE FOR EXPOSURE, published on Nov. 4, 2004 discloses a system for bandwidth control that employs a stored record of the spectral quality of portions of the bean exiting a seed laser, at least two distinct portions, and perhaps more, based on pulse repetition rate for the laser system. The bandwidth control system, based on actual operating pulse repetition rate and the stored beam profiles then selects enough portions of the beam with the smallest bandwidth to adequately seed the amplifier portion of the laser system. The selection may be made with what are basically shutters the allow only selected portions of the beam to pass to the amplifier portion of moveable mirrors to change the path between an output aperture on the seed laser and an input aperture on the amplifier portion to pick the portion of the beam entering the amplifier, or a combination of both. This possibly changes with a pulse repetition rate change.
The recent work of Huggins et al., “Effects of laser bandwidth on OPE in a modern lithography tool.”, Optical Microlithography XVIII (2006), describes controlling bandwidth to effect photolithographic process variations.
Active bandwidth control as proposed herein using a variable aperture external of the laser resonance cavity and in combination with other bandwidth control actuators has various advantages over the art. The optics within the cavity can be simplified and reduced in number, particularly in a single chamber or in the amplifier of a multiple changer laser system where optical loading is most severe. The variable aperture, in combination with other actuators can be either a coarse or fine adjuster for bandwidth control.